1. Field of the Invention
This document relates to a display apparatus, and more particularly, to an image processing apparatus and method of a plasma display panel.
2. Background of the Related Art
In general, a plasma display apparatus comprises a plasma display panel, i.e., a light-emitting device. The plasma display apparatus is called a flat display apparatus that displays a motion picture or a still image using a gas discharge phenomenon within the plasma display panel.
In the plasma display panel, a plurality of first and second sustain electrode lines and address electrode lines are formed in upper and lower glass substrates, respectively. The entire screen is divided into a plurality of cells by means of the respective electrode lines. Images are displayed by an address discharge and a sustain discharge that selectively occur within each cell.
The term “address discharge” refers to a discharge between the address electrode and the sustain electrode, and the term “sustain discharge” refers to a discharge between the first and second sustain electrodes. The sustain discharge functions to sustain an address discharge.
FIG. 1 is a schematic view illustrating a general plasma display apparatus. As shown in FIG. 1, a plasma display panel 10 comprises 640 R (Red), G (Green) and B (Blue) address electrode lines R1, G1, B1, . . . , R640, G640, B640 (hereinafter referred to as “vertical electrode lines”), and 480 first and second sustain electrode line pairs S1, S2, . . . , S479, S480 (hereinafter referred to as “horizontal electrode lines).
A microcomputer 20 digitizes externally input R, G and B image data and outputs 8-bit R, G and B digital image data (implement 256 gray levels). The microcomputer 20 also outputs various control signals necessary to drive the plasma display panel 10 according to the digital image data and an external signal.
A scan and sustain driver 30 supplies the 480 horizontal electrode lines S1 to S480 with a scan pulse and sequentially scans the 480 horizontal electrode lines one by one, according to the control signal of the microcomputer 20. The scan and sustain driver 30 then supplies the entire horizontal electrode lines S1 to S480 with a sustain pulse in order to sustain a discharge and emission of each cell.
The scan and sustain driver 30 comprises a clock and data generator 31 that generates a clock signal (CLK) and a data signal (DO) according to the control signal of the microcomputer 20, a sustain pulse generator 32 that generates the sustain pulse according to the control signal of the microcomputer 20, and a driving Integrated Circuit (IC) 33 connected to the 480 horizontal electrode lines S1 to S480, for sequentially supplying the scan pulses to the 480 horizontal electrode lines S1 to S480 and then supplying the sustain pulse to them at the same time, according to the clock signal (CLK), the data signal (DO) and a sustain pulse.
A memory unit 40 stores the R, G and B digital image data, which are output from the microcomputer 20, on a frame basis, a color basis and a bit basis. An address driver 50 reads bit values of 640 R, G and B digital image data corresponding to the horizontal electrode lines scanned by the scan and sustain driver 30 from the memory unit 40, and supplies the read bit values to the 640 R, G and B vertical electrode lines R1 to B640.
Meanwhile, a method of driving the plasma display apparatus can be mainly classified into a sub-field driving method and a sub-frame driving method. A process in which the plasma display apparatus constructed above displays an image of 256 gray levels on the plasma display panel screen in accordance with the sub-field driving method will be described below.
In the sub-field driving method, to implement 2x gray levels, one frame screen is displayed with it being divided into X sub-field screens. Externally input image data are digitized into X-bit digital image data and then supplied to the plasma display panel.
Furthermore, each sub-field screen consists of a reset period, an address period and a sustain period. Of them, the reset period and the address period are allocated in the same manner every sub-field, but the sustain period is differently allocated depending on bit weight of digital image data displayed in the address period. Therefore, gray levels of an image can be implemented through a combination of the respective sub-fields (using an eye's integral effect).
That is, as shown in FIG. 2, one frame is divided into eight sub-fields (SF1 to SF8). If a luminance value corresponding to 128:64:32:16:8:4:2:1 is made to correspond to each sub-field, an image corresponding to gray level data 0 to 255 can be displayed through a combination of several sub-fields.
Therefore, the microcomputer 20 digitizes the externally input R, G and B image data in order to implement 256 gray levels, outputs 8-bit R, G and B digital image data (the highest bit value B1 to the lowest bit value B8). The microcomputer 20 also outputs various control signals necessary to drive the plasma display panel 10 according to the digital image data and an external signal.
At this time, the 8-bit R, G and B digital image data output from the microcomputer 20 are stored in the memory unit 40 on a frame basis, a color basis and a bit basis.
Thereafter, in the reset period and the address period of the first to eight sub-field screens (SF1 to SF8), the driving IC 33 applies an erase pulse for erasing wall charges formed in a previous field to the entire horizontal electrode lines S1 to S480 (first step). The driving IC 33 applies a write pulse for forming uniform wall charges to a three-electrode surface discharge type plasma display panel 10 (second step). The driving IC 33 applies an erase pulse again to form wall charges on the 640 R, G and B vertical electrode lines R1 to B640 so that a voltage of a subsequently applied address pulse is lowered (third step). If the scan pulses are sequentially applied to the 480 horizontal electrode lines S1 to S480 one by one according to the clock signal (CLK), the data signal (DO) and the sustain pulse (fourth step), scanning of the 480 horizontal electrode lines S1 to S480 is completed.
Furthermore, when supplying the scan pulse at the fourth step, the address driver 50 supplies each of the 640 R, G and B vertical electrode lines R1 to B640 with an address pulse (1 bit value of R, G and B digital image data) corresponding to a horizontal electrode line, which is scanned as the scan pulse is applied, in synchronization with the scan signal. Therefore, a discharge can be generated within a discharge space of each cell to which an address pulse of logic “high” is applied.
At this time, the address driver 50 disposes the 8-bit R, G and B digital image data (B1 to B8) corresponding to each cell in the form of B1→SF1, B2→SF2, . . . , B7→SF7, B8→SF8.
Meanwhile, if the address period of each of the sub-field screens (SF1 to SF8) is completed, the driving IC 33 receives a sustain pulse from the sustain pulse generator 32 and supplies the entire horizontal electrode lines S1 to S480 with a sustain pulse whose number is proportional to SF1:SF2: . . . SF7:SF8=27:26: . . . 21:20 (a relative luminance ratio). That is, during a time T, the most significant bit (MSB) scans lower bits in order of bits close to the MSB, during T/2, T/4, . . . T/64, T/128, so that a discharge and emission of some cells in which a discharge has occurred in the address period is sustained during a period where the sustain pulse is supplied (the sustain period).
If the configuration of the first to eight sub-field screens (SF1 to SF8) is completed through the above process, an image of 256 gray levels is displayed on the plasma display panel 10.
Furthermore, the number of sub-fields is generally about 12. The number of gray levels that can be obtained using two sub-fields is 2. However, all of them cannot be used. This is because of pseudo contour in a motion picture, which has been a problem depending on the sub-field driving method of the plasma display panel.
If a specific combination of sub-fields for producing a lot of pseudo contour is all taken out, the number of real gray levels that can be really produced can be less than 100.
To represent gray levels higher than that using such a small gray level, another method has to be additionally used. This method is an intermediate gray level generating method called “half-toning”. This method functions to fill between real gray levels.
Error diffusion or dithering is usually used. The error diffusion method is a slightly modified method of the Floyd Steinberg method. The dithering method is a cyclic repeat method on a frame cycle using a 4×4 mask.
The error diffusion method is a method of diffusing gray level error values of an input pixel into neighboring cells. This method is technology that is generally used in a printing apparatus such as a printer. More particularly, if the error diffusion method is employed in an AV motion picture, excellent representation is possible. In this method, predetermined pattern noise, which is generated due to the use of a dither, does not occur.
If a still image such as PC mode is to be displayed, however, the error diffusion method has a still noise pattern. If a luminance difference between real gray levels is high, the noise pattern is very strong. Therefore, this may result in dot noise of a pattern. More particularly, this is true of a dark screen on which low gray levels are generally displayed.
To prevent the still noise pattern from occurring, a method of adding some random noise to an image signal is generally used. If the random noise is used, the noise pattern is changed little by little every moment. This removes the dot noise of the still noise pattern. However, the noise pattern that continues to move can be seen as noises in a dark image having lots of low gray level. This may lead to a sizzling phenomenon of the screen. This cannot be avoided even if a very small amount of noise is added.
More particularly, in the plasma display panel, in the case of a screen having a low Average Picture Luminance (APL), a luminance difference between real grays is high since a lot of sustain pulses is used. Therefore, there is a problem in that the sizzling noise is very unpleasant to the eye.